1. Field of the Invention
This invention relates to a two-level threshold circuitry and more particularly to such circuitry for large scale digital memories such as found in LSI and VLSI semiconductor devices.
2. Description of the Prior Art
As the packing density of large scale and very large scale integrated circuitry becomes greater and greater, the cost of the individual circuit units becomes more and more inexpensive. This is particularly true in regard to digital memories which are currently being built with hundreds of thousands of memory cells and eventually millions of memory cells, each one of which stores an individual bit of data. As the size of the respective memories increases, the capacitances of the bit lines or sense lines increases with the result that it is more difficult to rapidly change the voltage levels on those lines. This is particularly true of read-only memories (ROMs) and programmable read-only memories (PROMs) which are finding increasing usage as the cost per unit cell decreases.
ROMs were initially in the form of diode and core matrices and were employed as data memories. However, in 1951, Maurice V. Wilkes postulated a decoder for a digital computer which was formed of a fixed matrix (a ROM) wherein selected nodes of the matrix were provided with core crossconnections so that, for any given set of incoming signals, a desired set of output signals could be generated (see Wilkes, "The Best Way to Design an Automatic Calculating Machine", report of Manchester University Computer Inaugural Conference, July, 1951, page 16). With the advancement of integrated circuitry, such core matrices are now created in the form of IC ROMs which again can have tens of thousands of gates to accommodate an extremely large number of boolean operations to be performed in the sense that the incoming signals are merely an address to a particular location in the ROM at which are stored the desired combination of output signals required.
Wilkes' original design led to the concept of microprogramming as a means of decoding computer instructions, (see Faber et al U.S. Pat. No. 3,983,539 or Ferguson et al U.S. Pat. No. 3,886,523). However, the concept has been employed for addition, subtraction, and multiplication through the use of tables in the form of such matrices or memories, (see Gerace, "Microprogram Control for Computing Systems", IEEE Transactions on Electronic Computers, December, 1963, at page 742). This concept has now advanced to what is today referred to in many circles as stored logic or structural firmware as well as control firmware. This in turn gives the computer designer greater flexibility in the designing of various functions that he wishes the circuitry to perform and tends to create a greater demand for larger and larger memories or IC memories with higher packing densities.
As the packing density of integrated circuit devices becomes higher, there is corresponding increase in the switching speed of the various circuits. However, particularly in ROM type memories, the increase in the number of gates in the circuitry adds to the capacitances of the sense lines which tends to impede the switching speed.
It is, then, an object of the present invention to provide an improved threshold sensing device for large scale integrated circuit memories.
It is another object of the present invention to provide improved threshold devices for both ROMs and PROMs.
It is still a further object of the present invention to provide an improved threshold circuitry for large scale integrated circuits which can be switched between two different thresholds.